I know basic about URL, URN, and URI; but little on their differences. The thing I can't understand is in a page like: http://www.bernzilla.com/item.php?id=100 which part is URL, URN and URI!
URI is whole part but URL and URN??

Picture from wikipedia

Edit:
from w3c.org:

An URL is a type of URI that identifies a resource via a
representation of its primary access mechanism (e.g., its network
"location"), rather than by some other attributes it may have.

3 Answers
3

Contains information about how to fetch a resource from its location. For example:

http://example.com/mypage.html

ftp://example.com/download.zip

mailto:user@example.com

file:///home/user/file.txt

tel:1-888-555-5555

http://example.com/resource?foo=bar#fragment

/other/link.html (A relative URL, only useful in the context of another URL)

URLs always start with a protocol (http) and usually contain information such as the network host name (example.com) and often a document path (/foo/mypage.html). URLs may have query parameters and fragment identifiers.

urn:publishing:book - An XML namespace that identifies the document as a type of book.

URNs can identify ideas and concepts. They are not restricted to identifying documents. When a URN does represent a document, it can be translated into a URL by a "resolver". The document can then be downloaded from the URL.

An example of a URI that is neither a URL nor a URN would be a data URI such as data:,Hello%20World. It is not a URL nor a URN because the URI contains the data. It neither names it, nor tells you how to locate it over the network.

There are also uniform resource citations (URCs) that point to meta data about a document rather than to the document itself. An example of a URC would be an identifier for viewing the source code of a web page: view-source:http://example.com/. A URC is another type of URI that is neither URL nor URN.

Frequently Asked Questions

I've heard that I shouldn't say URL anymore, why?

The W3 spec for HTML says that the href of an anchor tag can contain a URI, not just a URL. You should be able to put in a URN such as <a href="urn:isbn:0451450523">. Your browser would then resolve that URN to a URL and download the book for you.

Do any browsers actually know how to fetch documents by URN?

Not that I know of, but modern web browser do implement the data URI scheme.

Does the difference between URL and URI have anything to do with whether it is relative or absolute?

No. Both relative and absolute URLs are URLs (and URIs.)

Does the difference between URL and URI have anything to do with whether it has query parameters?

No. Both URLs with and without query parameters are URLs (and URIs.)

Does the difference between URL and URI have anything to do with whether it has a fragment identifier?

No. Both URLs with and without fragment identifiers are URLs (and URIs.)

But doesn't the W3C now say that URLs and URIs are the same thing?

Yes. The W3C realized that there is a ton of confusion about this. They issued a URI clarification document that says that it is now OK to use the terms URL and URI interchangeably (to mean URI). It is no longer useful to strictly segment URIs into different types such as URL, URN, and URC.

Can a URI be both a URL and a URN?

The definition of URN is now looser than what I stated above. The latest RFC on URIs says that any URI can now be a URN (regardless of whether it starts with urn:) as long as it has "the properties of a name." That is: It is globally unique and persistent even when the resource ceases to exist or becomes unavailable. An example: The URIs used in HTML doctypes such as http://www.w3.org/TR/html4/strict.dtd. That URI would continue to name the HTML4 transitional doctype even if the page on the w3.org website were deleted.

Thanks for answer. I edited my question. can you take look again?
–
undoneSep 2 '11 at 13:35

I don't understand that quote about describing applications and I can't see a comment where someone points that out. I'll update my answer to describe the separate parts.
–
paulmorrissSep 2 '11 at 13:53

@paul don't you mean URI in the first line? id=100 isn't a location, it's an indicator.
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DisgruntledGoatSep 7 '11 at 14:37

I think I meant URL. With some sites if you change the id you get different "pages" in normal-speak. Reading the definition of resource on wikipedia I think a page is an example of a resource.
–
paulmorrissSep 7 '11 at 14:43